This invention relates to starch-based corrugating adhesives and methods for their preparation. The invention is also concerned with methods of uniting articles, particularly laminated articles of fibrous, cellulosic webs, and more particularly, with the production of paperboard for use in the production of corrugated containers. The invention is also concerned with articles produced through the use of the new adhesive compositions.
One usual method of making corrugated board consists of corrugating a strip of paper by means of a corrugated roller, applying an adhesive to the tips of the flutes of the corrugations on one side and adhering another strip of paper called a "liner", to the flute tips by use of heat and under high pressure.
This product, called "single face" corrugated board may be used as is. However, it is usual to make "double face", also called "double back" board by sending the single face board into the second stage of the corrugating machine called the "double backer" stage. The term "single facer" generally refers to the first stage of the machine. An adhesive is applied to the opposite flute tips, and a second liner of paper is adhered to said opposite side by the use of heat and under a relatively small amount of pressure. The fact that the use of a great amount of pressure in the adherence of the second strip of paper would tend to crush the corrugations makes the adhesive problem in the second step quite difficult.
Starch-based corrugating adhesive formulations have been in wide use since the advent of the Stein-Hall technology, as disclosed in U.S. Pat. No. 2,051,025, granted Aug. 18, 1936, to J. V. Bauer, and U.S. Pat. No. 2,102,937, granted Dec. 21, 1937, also to J. V. Bauer.
A Stein-Hall type adhesive is a two component aqueous system. One component in this system is generally formed from a cooked, or gelatinized starch material, which serves as a carrier phase. Other materials which can form relatively viscous aqueous slurries, such as carboxymethyl cellulose, may also be used as a carrier phase.
The second component or phase is formed from a raw, ungelatinized starch material. This second phase is a latent or potential adhesive phase. That is, the adhesive characteristics of such a formulation are not fully developed until after the adhesive has been applied to the tips of the flutes of the corrugated web, the liner has been pressed against the adhesive-coated flutes, and heat and pressure have been applied; the heat causing the granular starch material to gelatinize and develop structure.
The swelling and gelatinization of the latent adhesive phase takes place as the newly assembled corrugated board is passed through a hot plate-dryer system that is associated with a corrugating machine. This system also partially dries the corrugated board and sets the adhesive sufficiently so that it can be subjected to subsequent operations, such as trimming, slitting, and sheeting without delamination.
The initial degree of cohesiveness in the bond of the corrugated board is referred to as the green bond strength. This characteristic determines the ability of the newly formed corrugated board to resist the instantaneous high shear forces that are developed during subsequent processing operations, such as the trimming, slitting, and sheeting operations, and is not necessarily an indication of the final bond strength.
Green bond strength, or for brevity, green strength, is a major limiting factor which controls the operating speed at which corrugated board may be manufactured on a given machine. As experienced machine operator often will operate a machine at the maximum speed of which it is capable without having the board delaminate during the trimming, slitting, and sheeting operations. Since the resistance to delamination is a characteristic that is directly dependent on green strength, the characteristics of the corrugating adhesive play a direct role in machine efficiency, which in turn determines the rate of return on the fixed investment represented by the machine.
After corrugated board has been trimmed, slit, and sheeted, it is stacked and sent to storage, where the adhesive cures to a full strength. Until the bond is dry and fully cured, the corrugated board may be delaminated by slowly and firmly pulling the liner away from the corrugated sheet.
Since the middle thirties, one of the major advances in corrugating adhesive technology is that disclosed in U.S. Pat. No. 3,355,307, granted Nov. 28, 1967, to John J. Schoenberger and Raymond P. Citko. This patent discloses a single phase corrugating adhesive referred to as a "no-carrier" system, in which partially swollen starch granules are present as a homogeneous phase, suspended in an aqueous, alkaline vehicle. The elimination of the carrier phase permitted substantial operating economies. The no-carrier type corrugating adhesive disclosed and claimed in the Schoenberger-Citko patent was applied and cured in the same manner as the Stein-Hall type adhesive formulations.
Other advances in the corrugating adhesive field related primarily to building in moisture resistance, generally through the addition of synthetic resins, such as urea-formaldehyde, phenol-formaldehyde, and resorcinol-formaldehyde resins.
The Bauer patents represented a significant advance in the art, for their time. Bauer reported that the use of his adhesive systems permitted corrugated board machines to be operated up to 20% faster than would have been possible utilizing prior art adhesive compositions. The prior art compositions included preparations based on sodium silicate and adhesive preparations based on the use of gelatinized starches or modified starches such as dextrins.
In the Stein-Hall type system, according to the teachings of Bauer, the time required to form an adhesive bond between the corrugated interliner and a liner or facer sheet is substantially independent of the rate at which the moisture in the adhesive is driven off by heat or absorbed by the paper. According to Bauer, the time required to form the adhesive bond depended mainly on the time required to cause gelatinization of the granular starch particles suspended in the gelatinized starch carrier phase. As this granular starch gelatinized, water was taken up, and the viscosity of the adhesive was raised rapidly, to form an immediate green bond. Bauer considered that tapioca, rye and potato starches were inherently superior to corn, wheat, and rice starches for making corrugating adhesives in accordance with his developments.
Bauer described four principal factors that he considered to determine the suitability of the starch for use as the latent adhesive component in his system. These four factors were:
1. the time required for complete gelatinization of the granular starch by means of the heat input on the corrugating machine. PA1 2. The temperature at which the starch gelatinizes. PA1 3. the viscosity developed after gelatinization. PA1 4. the degree of tackiness developed after gelatinization
However, subsequent developments indicate that the green bond strength depends significantly on the evaporation of moisture from the bond site; Thayer & Thomas, Analysis Of the Glue Lines in Corrugated Board; TAPPI; 22nd Corrugated Containers Conference, May 1971.